agp/intel: Fix wrong kunmap in i830_cleanup()

Add a missing NULL check and fix the wrong address passed to kunmap()
in i830_cleanup().

Cc: stable@kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[danvet: added cc stable]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/char/agp/intel-gtt.c

@@ -812,8 +812,10 @@ static int intel_fake_agp_fetch_size(void)
 
 static void i830_cleanup(void)
 {
-	kunmap(intel_private.i8xx_page);
-	intel_private.i8xx_flush_page = NULL;
+	if (intel_private.i8xx_flush_page) {
+		kunmap(intel_private.i8xx_flush_page);
+		intel_private.i8xx_flush_page = NULL;
+	}
 
 	__free_page(intel_private.i8xx_page);
 	intel_private.i8xx_page = NULL;